A digital image cannot be rendered in units smaller than pixel units, thus causing jaggies at the contour (edge) of a polygon. For example, when a polygon as shown in (A) of FIG. 16 is rendered, a scaled-up oblique contour of the polygon has stair-stepping (jagged) appearance as shown in (B) of FIG. 16. A technique of anti-aliasing is used to make such jaggies less prominent.